The present invention relates to a method for determining the presence of leaks in a system with flowing fluid and an apparatus for the same.
It is known with regard to systems with flowing fluid to determine the flow rate and/or flow velocity into the system at the input and out of the system at the output in order to control the flow of the fluid through the system with these determined values.
For example, in dialysis devices, among other things, the flow velocity of the dialysis fluid is detected both before the dialyser and after the dialyser in order to control the flow of the dialysis fluid through the dialyser, and therefore the dialysis itself.
Accordingly, it is known from International Application No. WO 95/22743 to arrange a first sensing means comprising a throttle, a pump and a pressure sensor both before and after the dialyser in the dialysis fluid conduit, the pressure sensor thereby measuring the pressure between the throttle and the pump. Thus, on the one hand as a result of the relationship between pressure and flow velocity at the throttle, a constant rate of flow can be set by maintaining a constant pressure with the pump. On the other hand, the flow velocity before and after the dialyser can be calculated with the aid of the measured pressure values, and compared.
In addition, second sensing means are arranged between the pump and the dialyser for precisely measuring the flow velocity both before and after the dialyser. In this way the ultrafiltration may be exactly defined by comparing the measured values, the difference between the values thus providing the rate of ultrafiltration.
In order to permit exact and reliable control of the dialyser fluid through the dialyser, as well as precise ultrafiltration, the sensing means before and after the dialyser must be calibrated against one another.
This is usually carried out with a constant dialysis fluid rate through the dialyser apparatus with the dialyser disconnected, the sensing means thereby being calibrated against one another such that they indicate the same value. For example, the dialysis treatment, which generally lasts about four hours, is interrupted briefly every thirty minutes for such calibration.
When a leak is present, either dialysis fluid will escape or air will enter from outside. In this case the sensing means before and after the dialyser detect or measure different values, respectively, it being impossible to determine whether these different values result from leaks or from variations within the sensing means themselves.
Thus, when a leak is present, if the sensing means are calibrated against one another such that they indicate the same value, although they actually detect or measure different dialysis fluid currents, this can lead to control defects.
This is dangerous, particularly for dialysis machines where it is important that the fluid currents supplied to and discharged from the dialyser are precisely tared. As a result of deposits, specifically on the sensing means that are connected downstream of the dialyser and in contact with the contaminated fluid in the dialyser, the detection becomes more and more imprecise with increasing operational time. For example, proteins, urea, cholesterols and the like, which have been removed from the blood in the dialyser, can be deposited at that point. Consequently the sensing means are calibrated against one another in a regularly repeated taring phase, for which the first sensing means connected upstream of the dialyser deliver(s) the reference levels. The first sensing means are in contact only with fresh dialysis fluid so that deposits, for example of the above-mentioned substances, and the resulting increasing imprecision in operation, are improbable.
The above-mentioned fat and protein deposits occurring essentially at the downstream sensing means can be dissolved and flushed out with highly alkaline sodium carbonate or other suitable means so that, for cleaning purposes, sodium carbonate, for example, is advantageously passed through the dialyser apparatus at regular intervals.
However, it is also possible that, when using a dialysis fluid containing bicarbonate, as well as calcium carbonate among other components, which under certain conditions can be precipitated out of the dialysis fluid, will be deposited on both sensing means. These calcium deposits can be removed easily with an acid, such as citric acid, for example. Therefore, an acid that dissolves and flushes away these calcium deposits is likewise advantageously passed through the apparatus at regular intervals for cleaning purposes.
The precision of the dialysis process can be maintained over a long period of time by means of the calibration and the described cleaning procedure, provided that a leak does not occur. If, for example, dialysis fluid were to escape from the system as a result of a leak, the second sensing means will indicate a lower value than that indicated by the first sensing means. This value corresponds to the actual flow, however, during taring it would be presumed that deposits were present on the second sensing means and the second sensing means would be calibrated to the higher value supplied by the first sensing means.
A consequence of this is that, for example, during ultrafiltration, a lower quantity of fluid than necessary is extracted, which can lead to severe complications for the patient.
A method for determining blood leaks in a dialyser during a high flux hemodialysis treatment is known from International Application No. WO 97/11771. If the pressure on the blood side sinks during the treatment below the pressure on the dialysate side, the dialysate flow is halted and the rate of ultrafiltration is increased so that the pressure on the blood side becomes positive relative to the pressure on the dialysate side. In this way, in the event of a leak, blood arrives on the dialysate side, and upon recommencing the dialysis flow is conducted to a blood leak detector. In this manner it is possible to detect blood leaks in the membrane of the dialyser, however other leaks in the dialyser or in the conduits leading to the dialyser cannot be determined with this apparatus.
U.S. Pat. No. 5,350,357 describes an apparatus for peritoneal dialysis with a pump apparatus for pumping the dialysate, the pump apparatus comprising a diaphragm. This is operated with fluid pressure or pressurised air, as are valves for controlling the dialysate flow. In order to determine whether the pump apparatus or the valves have leaks a positive and negative air pressure is alternately applied in a test phase. If the applied pressure falls or rises during a predetermined time period above or below a predetermined value, an error indication is generated. This allows the pump apparatus, and the control valves, to be checked for leaks. However, it cannot be determined e.g. if the dialysis conduits have a leak.
It is known from European Patent No. 298,587 to arrange two flow meters both in the dialysate conduit before the dialyser and in the dialysate conduit after the dialyser. These are calibrated against one another in a calibration phase so that each pair indicates the same value. If these values should then deviate from one another during operation an alarm is generated. However, it is not ascertainable whether the deviations result from a leak, from errors in the flow meters, or from deposits on the measurement elements of the flow meters.
In view of this background an object of the present invention according to a first aspect is to provide a method and an apparatus with which leaks can be determined in a system with flowing fluid.
According to a second aspect it is another object of the present invention to provide a method and an apparatus with which it can be determined whether a leak exists or whether deposits are present on the sensing means, in order to ensure an exact calibration of the sensing means, particularly for dialysis apparatus.
In accordance with the present invention, these and other objects have now been realized by the invention of a method for calibrating a sensor for flowing fluid in a system including a dialysis monitor, a fluid input, and a fluid output, the method comprising applying a first pressure to the flowing fluid and detecting a first flow of the flowing fluid at the fluid input and the fluid output, applying a second pressure to the flowing fluid and detecting a second flow of the flowing fluid at the fluid input and the fluid output, and determining whether any detected differences between the first and second flows are the result of leakage in the system are caused by the sensor. In a preferred embodiment, the method includes determining a first difference between the first flow of the flowing fluid at the fluid input and the second flow of the flowing fluid at the fluid input, determining a second difference between the first flow of the flowing fluid at the fluid output and the second flow of the flowing fluid at the fluid output, determining a third difference between the first difference and the second difference, determining a fourth difference between the first flow of the flowing fluid at the fluid input and the first flow of the flowing fluid at the fluid output, and determining a fifth difference between the second flow of the flowing fluid at the fluid input and the second flow of the flowing fluid at the fluid output. Preferably, the method includes determining whether the third difference is not zero, whereby the determined difference is caused by leakage in the system. In another preferred embodiment, the method includes determining whether the third difference is zero, and the fourth difference or the fifth difference is not zero, whereby the determined difference is caused by the sensor.
In accordance with one embodiment of the method of the present invention, applying of the first pressure to the flowing fluid is carried out at a first constant fluid flow rate and applying of the second pressure to the flowing fluid is carried out at a second constant fluid flow rate. In a preferred embodiment, the first and second constant fluid flow rates are the same.
In accordance with another embodiment of the method of the present invention, both the first pressure and the second pressure are either a positive pressure or a negative pressure.
In accordance with another embodiment of the method of the present invention, one of the first pressure and the second pressure comprises a positive pressure and the other of the first pressure and the second pressure comprises a negative pressure.
In accordance with another embodiment of the method of the present invention, the flowing fluid comprises a dialysis fluid.
In accordance with the present invention, apparatus has also been discovered for calibrating a sensor for flowing fluid in a system including a dialysis monitor, a fluid inlet, and a fluid outlet, the apparatus comprising at least one first detecting means for detecting a first flow of the flowing fluid at the fluid input, at least one second detecting means for detecting a second flow of the flowing fluid at the fluid output, pressure means for applying a first pressure to the system in a first phase and a second pressure to the system in a second phase, each of the first and second pressures comprising either a positive pressure or a negative pressure, and the first and second pressures being different pressures, and evaluation means for evaluating the first and second flows. In a preferred embodiment, the evaluation means includes means for providing a first difference between the first flow of the flowing fluid at the fluid input and the second flow of the flowing fluid at the fluid output, a second difference between the first flow of the flowing fluid at the fluid output and the second flow of the flowing fluid at the fluid output, a third difference between the first difference and the second difference, a fourth difference between the first flow of the flowing fluid at the fluid input and the first flow of the flowing fluid at the fluid output, and a fifth difference between the second flow of the flowing fluid at the fluid input and the second flow of the flowing fluid at the fluid output, and determining means for determining whether the differences are the result of leakage in the system or are caused by the sensor. In a preferred embodiment, the determining means determines that the determined difference is caused by a leak when the third difference is not zero. In another embodiment, the determining means determines that the determined difference is caused by the sensor when the third difference is zero and the fourth and fifth differences are not zero.
In accordance with one embodiment of the apparatus of the present invention, the evaluation means comprises a throttle, a pump, and a pressure sensor whereby a pressure can be applied to the flowing fluid thereby.
One embodiment of the present invention is achieved in a method, wherein
in a first phase a first pressure is applied to the fluid and the rate of flow and/or flow velocity is detected at least at the input into the system and at the output out of the system,
in a second phase a second pressure that differs from the first pressure is applied to the fluid and the flow rate and/or flow velocity is detected at least at the input into the system and at the output out of the system,
and an indicator is formed from the detected values which indicates a leak in the system.
In this way it is possible to recognise a leak in a system with flowing fluid in a simple manner, because two different pressures are applied successively to the fluid in only one check phase, and the values detected in this way by the sensing means indicate a leak. If the values determined in the first phase and in the second phase at the input and output deviate from one another, then a leak is present.
The term xe2x80x9cleakxe2x80x9d is intended to signify every kind of leak that can arise in a system with flowing fluid. For example, this can concern leaks that are in valves present in the system and allow fluid to move undesirably from one system area to the next one, but do not allow fluid to escape from or enter the system. Systematic leaks which arise particularly in systems with rigid walls when the latter have holes or tears and permit fluid to flow into or out of the system can likewise be concerned. However, non-uniform leaks which arise specifically in systems with elastic walls when the latter have tears or the like can also be concerned. In that case the tears can close at one pressure owing to the given elasticity of the walls while opening at another pressure, so that discontinuous leaks result.
Preferably a first difference is formed between the values determined in the first phase and in the second phase at the input, a second difference between the values determined in the first phase and the second phase at the output, and a third difference between the first and second differences. The third difference is the indicator for a leak in the system, a leak having occurred when the third difference is not equal to zero.
In this way it can be ascertained in a simple manner using a single value, whether a leak is present in the system, so that appropriate measures may be subsequently taken to deal with the leak.
Another object of the present invention is achieved in a method wherein,
a fourth difference is determined between the values determined at the input and the output in the first phase,
a fifth difference is determined between the values determined at the input and the output in the second phase, and
a discriminator indicates whether the deviations of the detected values are the result of a leak or are caused by the sensing means. In this regard the deviations are due to the sensing means when the third difference equals zero and the fourth or fifth difference is not equal to zero.
In this manner it can be ascertained whether the values determined in both phases can, for example, be used for calibration of the sensing means against one another so that an exact calibration can be ensured.
If a fluid flow of, for example, 500 ml/min is passed through the system, 20 ml/min of fluid will for example exit at the location of a leak upon applying a first predetermined positive pressure, so that at the output of the system only 480 ml/min will be determined. If in the second phase a higher second pressure is applied to the flowing fluid, an increased quantity of fluid will exit, for example 40 ml/min. Thus, at the output a fluid quantity of only 460 ml/min will be measured coming out of the system when 500 ml/min of fluid is again passed through the system. In this example the difference at the input amounts to 0 ml/min, while it amounts to xe2x88x9220 ml/min at the output. The third difference between these two values likewise amounts to xe2x88x9220 ml/min and therefore indicates that a leak is present in the system. If the deviating value determined at the output in the first phase were the result of an error in the sensing means and not of a leak in the system this deviating value would also occur upon applying the higher pressure in the second phase. In this case the difference at the output of the system would then be equal to zero, so that the third difference would be equal to zero and would indicate that no leak is present. In this case it would only be necessary to calibrate the sensing means at the input and the output of the system against one another, as the values at the output deviate from those determined at the input in both the first and second phases.
It is not necessary that the fluid flow which is passed through the system in the first phase be equal to the fluid flow passed through the system in the second phase, even though this is provided to simplify the investigation of leaks according to a preferred embodiment hereof. It is only necessary that the fluid flow passed through the system in each phase is constant so as to enable an exact prediction regarding the presence of leaks in the system to be made. For example, if a fluid flow of 500 ml/min is passed through the system in a first phase and a value of 480 ml/min is indicated at the output of the system as a result of an error in the sensing means, a value of 460 ml/min will be indicated at the output in the second phase when a fluid flow of 480 ml/min is passed through the system. In this case the differences at the input into the system and at the output out of the system amount to xe2x88x9220 ml/min in both phases, so that the third difference derived from these is equal to zero. This indicates that the deviations are due to errors in the sensing means and not to leaks in the system, since the third difference is equal to zero and the fourth and fifth differences are not equal to zero.
It should be mentioned at this point that the so-called output of the system may comprise several individual outputs, and likewise, the so-called input may comprise several individual inputs. In such a case, the values detected at the individual outputs or the individual inputs, respectively, would be combined in each respective phase to a single output or input value, respectively, which would then be used to form the differences in the described fashion. However, for the sake of simplification only a single input or output will be referred to in the following, although several inputs or outputs, respectively, which together form the respective input or output of the system, are always also included therein.
If a leak is present which, in a first phase, allows 20 ml/min of fluid to exit with a predetermined first pressure and a fluid flow of 500 ml/min, 480 ml/min will be detected at the output of the system. This leak allows more fluid to exit in a second phase with a higher predetermined pressure. Thus, if only 480 ml/min of fluid is passed through the system in the second phase 30 ml/min of fluid, for example, will exit, and at the output of the system only 450 ml/min will be detected. Thus the difference at the input amounts to xe2x88x9220 ml/min, at the output to xe2x88x9230 ml/min and the third difference between these two amounts to xe2x88x9210 ml/min. Thus it is clearly indicated that a leak is present in the system.
There can also be a situation wherein the difference ascertained at the output is zero despite the presence of a leak in the system. For example, if a fluid flow of 480 ml/min is passed through the system in the first phase, and 20 ml/min of fluid exits through a leak at a first predetermined pressure, more than 20 ml/min of fluid will exit there at a second predetermined higher pressure. For a fluid flow through the system of, for example, 500 ml/min this can then lead to 40 ml/min of fluid exiting at the leak, so that a fluid rate of 460 ml/min will likewise be indicated at the output. Thus, the difference at the output is equal to zero while at the input it amounts to +20 ml/min. Consequently, the third difference derived from this is also +20 ml/min and hence, owing to its deviation from zero, clearly indicates the presence of a leak in the system.
The pressure applied to the fluid in the first phase can be a positive pressure, as can the pressure applied to the fluid in the second phase. Only the difference between the pressures in the first and second phases is essential, it being also possible when considering this requirement to apply a negative pressure to the fluid in both the first and second phases.
It is, however, also possible to apply a negative pressure in the first phase and a positive pressure in the second phase, or vice versa. If a fluid flow of, for example, 500 ml/min is passed through the system in the first phase and a positive pressure applied, fluid can flow out of a leak at a rate of 20 ml/min, for instance, so that at the output of the system only 480 ml/min is detected. If a negative pressure is now applied in the second phase with a fluid flow of 480 ml/min, fluid can enter the system through the leak so that at the output 500 ml/min could be detected, for example. With this example a first difference of xe2x88x9220 ml/min will be detected at the input into the system and a second difference of +20 ml/min at the output of the system so that the third difference resulting from these two differences is +40 ml/min. Here again the presence of a leak in the system is clearly indicated by the deviation from zero of the third difference.
It is, however, advantageous when a positive pressure is applied in both the first and second phases. In this way fluid will always exit from the system in the event of a leak which is advantageous, particularly for dialysis apparatus. Impurities from outside could then not penetrate into the dialysis apparatus which would be possible when applying a negative pressure in both phases due to the penetration of fluid through a leak.
One object of the present invention is realized in apparatus wherein
at least a first and second means for detecting the flow rate and/or flow velocity of the fluid are provided, the first means being arranged at the input of the system and the second means being arranged at the output of the system,
means for generating a negative and/or positive pressure are provided in the system, the means being formed such that a first pressure can be applied to the fluid in a first phase and a second pressure can be applied to the fluid in a second phase, the first pressure differing from the second pressure,
and means are provided for evaluating the flow rates and/or flow velocities detected by the sensing means.
In this way a device for determining the presence of leaks in a system with flowing fluid is provided at a very low cost. Only means for sensing the flow rate and/or flow velocity, means for generating a negative and/or positive pressure and means for evaluating the values supplied by the sensing means are required. It should be noted at this point that in a system with several inputs and/or outputs, at least one sensing means is provided at every input or output, respectively. The above-described input or output can consequently comprise several inputs or outputs, respectively, and the latter condition is considered comprised in any future reference to an input or output, respectively.
Advantageously the evaluating means are formed such that they
determine a first difference value between the values supplied by the first sensing means in the first pressure phase and in the second pressure phase,
determine a second difference value between the values supplied by the second sensing means in the first pressure phase and the second pressure phase, and
determine a third difference value between the first and second difference values, the third difference value being the indicator for the presence of a leak in the system, a leak being present when the third difference is not equal to zero.
In this manner the method described above can easily be applied, the third difference supplying a value, or being an indicator, by means of which it can be determined if a leak is present in the system, so that appropriate measures may subsequently be taken to deal with the leak.
Another object of the present invention is achieved in an apparatus wherein the evaluating means are formed such that they
determine a fourth difference between the values determined at the input and the output in the first phase,
determine a fifth difference between the values determined at the input and the output in the second phase, and
provide a discriminator that indicates whether the deviations of the detected values result from a leak or are caused by the sensing means. In this regard the deviations are due to the sensing means when the third difference is equal to zero and the fourth or fifth difference is not equal to zero.
In this manner it can be ascertained whether the values determined in the different phases can be used for calibration of the sensing means, as described extensively above, so that an exact calibration is ensured.
Hence, with little cost, apparatus for generating a discriminator is provided which unequivocally determines whether the deviations at the sensing means are the result of a leak, or are due to soiling or other defects of the sensing means.
Advantageously, the means for detecting the fluid flow rate and/or flow velocity each comprise a throttle apparatus, pump means and a pressure sensing means, and are arranged such that pressure can be applied to the fluid. On the one hand, this permits the flow velocity to be determined in a simple manner as a result of the above-mentioned relationship between pressure and flow velocity of the fluid at the throttle. On the other hand, the pressure applied to the fluid can be controlled just as simply with the pump means. To this end the throttle apparatus and the pump means are advantageously arranged such that the pump of the sensing means disposed at the input is arranged after the throttle apparatus, and the pump of the sensing means disposed at the output is arranged before the throttle apparatus. In both cases the pressure sensing means are arranged between the respective pump means and the throttle apparatus. The pressure in the system can be regulated in a simple manner by increasing or reducing the throughput of the respective pumps.